1. Field of the Invention
This invention relates generally to glass with high infrared transmittance and high visible light transmittance and, more particularly, to a high transmittance float glass having low iron and low manganese content.
2. Technical Considerations
Solar cells (photovoltaic cells) and solar mirrors are used in the field of electricity generation. Solar cells convert solar energy to electrical energy. Solar cells typically have a high transmittance cover plate, such as a glass cover plate, through which the solar energy passes to reach the interior of the solar cell. Solar mirrors are used to reflect solar energy. Solar mirrors typically have a protective glass substrate. Solar energy passes through the substrate to a reflective coating, which reflects the solar energy back through the glass substrate to direct the solar energy to a designated area.
The glass used for solar cells and solar mirrors preferably has a high transmission in the electromagnetic spectrum above 380 nanometers (“nm”), e.g. transmission above 90% in the visible and the infrared (“IR”) range. These articles also preferably have a low absorption, e.g. below 2%, in the visible and the IR ranges. The particular visible and IR range of the electromagnetic spectrum, and the peak transmission vary depending on the semi-conductor material of the photovoltaic cell. For example and not limiting to the discussion, for a silicon photovoltaic solar cell, the preferred visible and IR wavelength range is in the range of 380-1200 nm, and the peak transmission is at about 900 nm to 950 nm.
Generally, in the manufacture of float glass, glass batch materials are melted. The molten glass is fined and homogenized, and the fined homogenized molten glass is formed into a flat glass ribbon by controllably decreasing the temperature of the molten glass as it floats on a molten metal bath. Typical batch materials include sand, soda ash, limestone, dolomite, and salt cake. While soda ash and salt cake are naturally very low in iron content, the remaining materials, particularly sand, can have significant concentrations of iron unless they are chemically treated to remove the iron.
A problem with iron in glass is that, as a general rule, the higher the iron content (particularly FeO), the lower the light transmittance of the glass. For applications requiring high light transmission, special sand having naturally low iron content or sand that has been chemically treated to remove the iron, is used. However, this increases the expense of the resultant glass. Glass having a low total iron content expressed as Fe2O3, e.g. less than about 0.025% by weight (hereinafter also referred to as “wt %” or “wt. %”), is referred to conventionally as low iron glass. The iron is not added to the batch material intentionally but is present as an impurity in the ingredients of the batch material.
Even though the iron content is low in low iron glasses, for solar cells, it is desirable to reduce the weight percent of ferrous iron (Fe+2) in the glass as much as possible to maximize the transmission, and minimize the absorption of the glass in the visible and IR range of the electromagnetic spectrum. Iron in the ferric state (Fe+3) is a less powerful colorant than iron in the ferrous state and shifts the transmittance spectrum of the glass toward yellow and away from the usual green-blue effect of the ferrous iron in glass. Stated another way, increasing iron in the ferric state while decreasing iron in the ferrous state, increases the transmission, and decreases the absorption of the glass in the visible and the IR range.
One technique to reduce the weight percent of ferrous iron in the glass is to include an oxidizing agent in the glass batch materials. In the past, oxidizing agents such as NaNO3, CeO2, Sb2O3, and As2O3, have been added to the glass composition to reduce the amount of FeO. However, these previous oxidizing agents themselves have disadvantages that include processing, environmental and safety concerns. For example, NaNO3 poses the problem of NOx emissions and As2O3 is poisonous. Sb2O3 and As2O3 are incompatible with the float glass process due to reactions in the tin bath that cause gray color streaks in the glass. Glass having CeO2 has been found to “solarize” when exposed to sunlight for prolonged periods. By “solarize” or “solarization” is meant that exposing low iron glass having cerium oxide to sunlight causes the glass to change from a yellowish color to a bluish color due to the photo-oxidation of Ce+3 to Ce+4 and the photo-reduction of Fe+3 to Fe+2. Blue Fe+2 absorbs more light than the yellow Fe+3, which decreases the transmittance of the glass and reduces the electrical output of the solar cell.
As can now be appreciated, it would be advantageous to provide a low iron glass compatible with the float glass system that has low levels of iron in the ferrous state (Fe+2) and does not have the solarization problem associated with prior glass.